V(D)J Recombination
The vertebrate immune system is capable of identifying and responding to an almost infinite array of pathogens. This is made possible by the extreme diversity of antigen receptors expressed by B and T cells. The diversity of these receptors is created by a special recombination event that takes place within the nuclei of progenitor B and T cells. This event is known as V(D)J recombination, or sometimes somatic recombination. V(D)J recombination can be simplified as a random shuffling of exons that creates a novel DNA sequence that will be transcribed and translated into a novel antigen receptor. The discovery of V(D)J recombination won Susumu Tonegawa the 1987 Nobel Prize in Physiology. Creating Novel Antibodies Antibodies are proteins composed of two heavy chains and two light chains. Each pair of chains has a unique variable region that acts as the binding site, and a constant region that provides the structure of the antibody. Heavy chains are encoded by the Immunoglobulin heavy locus (symbol: IGH@, located on chromosome 14), light chains are divided into two sub-classes kappa and lambda, each encoded by their own gene locus (kappa on chromsome 2, lambda on chromsome 22) (RefSeq 2014). All three of these gene locuses contain multiple sequences that can be recombined into new combinations that will encode for a novel heavy or light chain. In B-cells, novel antibodies are created during maturation. First, the heavy chain locus recombines to form a novel heavy chain, then the light chain locus recombines to form a novel light chain. Thus, it takes two rounds of recombination to produce a novel antibody. When the antibody is complete, it is expressed on the B-cell surface and the B-cell migrates to the lymph nodes to await activation (Schatz et al 1992). Progenitor B-cells undergo V(D)J recombination as they mature in the bone marrow. V(D)J recombination is facilitated by V(D)J recombinase, a complex of enzymes homologous to a transposase, that cuts and pastes specific gene sequences into a new order that is eventually transcribed into an antibody chain. An antibody chain gene is composed of a variable region (V), a diverse region (D), a joining region (J), and a constant region ©. There is one constant region for each isotype of antibody (IgM, IgG, IgA, etc.). A novel combination of variable, diverse, and joining regions (one of each) is selected from multiple options of each type of region encoded within the chain locus. This "selection" is a recombination event that attaches the chosen variable, diverse, and joining regions with a constant region. The specific recombination event, that actual "shuffling" of genes, is carried out primarily by the Rag1 and Rag2 proteins (Rag= recombination activating gene) within the V(D)J recombinase complex. The Rag proteins bind a Recombination Segment Sequence (RSS) that precedes each variable, diverse, and joining region. To prevent the joining of two variable regions or two diverse regions, different forms of RSS's precede variable and joining regions. A twelve nucleotide RSS precedes variable regions and a twenty-three nucleotide RSS precedes joining regions. Rag proteins will only recombine a pair of regions with one 12 RSS and one 23 RSS; this is known as the "12 and 23 rule." Once the recombination is complete, other enzymes in V(D)J recombinase stabilize the gene structure and ligate gene fragments together. After one of each regions is recombined into a new sequence, the DNA is transcribed into an mRNA molecule that undergoes alternative splicing (generating additional diversity) and is eventually translated into an antibody chain. As there are many unique variable, diverse, and joining regions encoded within one immunoglobulin gene locus, each individual region may be very far away from other regions. Two regions are brought into proximity by forming a hairpin loop or a tangle that allows recombination to occur. Each RSS is slightly different from each other, allowing them to bind with slightly different affinities to each Rag protein. This drives the "randomness" of V(D)J recombination. The number of different variable, diverse, and joining regions alone is not enough to create sufficient diversity to all pathogens. To create additionally diversity, an enzyme called terminal deoxynucleotide transferase (TdT) adds additional nucleotides to variable and joining regions to create additional novel sequences. TdT is a special kind of DNA polymerase that does not require a template to add nucleotides to a sequence (Schatz et al 1992). Some recombination events produce antibodies that are capable of binding to host cells. In order to prevent cross-reactions of antibodies with host cells, B-cells that bind host antigens are negatively selected against so that they never enter circulation. References Schatz DG, Oettinger MA, Schlissel MS (1992). "V(D)J Recombination: molecular biology and regulation." ''Ann Rev Immunol. '' Hammosh A (2013). "607358: Autoimmune regulator (AIRE)." ''OMIM. '' RefSeq (2014). "Immunoglobulin lambda locus." ''Entrez gene. '' RefSeq (2014). "Immunoglobulin heavy locus." ''Entrez gene. '' Useful video: V(D)J Recombination II